Nowadays a wide variety of jacks are in use, such as those functioned by air pressure, oil pressure of mechanical means. However, air pressure jacks have a drawback of malfunction caused by air leaking and oil-pressure ones have a drawback of oil leakage caused by small ruptures of oil seals and ensuing blockage caused by dirt adhered to oil which has leaked out. Another common disadvantage of conventional jacks is their low speed in lowering down, after having been raised up.